The Bosphorus Bridge (which crosses the Bosphorus Strait) in Istanbul.

Photo: Reuters: Osman Orsal

Turkey has opened the world's first underwater rail link between two continents, connecting Asia and Europe in a project envisioned by Ottoman sultans more than a century ago.

Called the Marmaray, the engineering feat spans 13 kilometres some 60 metres below the Bosphorus Strait, and it will carry subway commuters in Istanbul and eventually serve high-speed and freight trains.

Turkish prime minister Tayyip Erdogan has referred to the Marmaray as the "project of the century" and says it fulfils an age-old "dream of our ancestors".

"Today we are realising the dreams of 150 years ago, uniting the two continents and the people of these two continents," Mr Erdogan said at the opening, which coincides with the 90th anniversary of the founding of the modern Turkish Republic.

Plans for a rail tunnel below the Bosphorus date to at least 1891, when Ottoman sultan Abdulhamid, a patron of public works whom Mr Erdogan frequently evokes, had French engineers draft a submerged tunnel on columns that was never built.

Japan and Turkey are the two wings of Asia. Let us dream together of a high-speed train departing from Tokyo, passing through Istanbul and arriving in London

Japanese PM Shinzo Abe

The gleaming Marmaray is an immersed tube set in the seabed and built by Japan's Taisei Corp with Turkish partners Nurol and Gama. The bulk of financing came from the Japan Bank for International Cooperation.

"Japan and Turkey are the two wings of Asia. Let us dream together of a high-speed train departing from Tokyo, passing through Istanbul and arriving in London," said Japanese prime minister Shinzo Abe, who attended the opening.

Erdogan's infrastructure legacy to change Turkey

At a cost of 5.5 billion lira ($2.91 billion), the project is one of Mr Erdogan's "mega projects," included in an unprecedented building spree designed to change the face of Turkey.

Other projects include a 50-kilometre canal to rival the Suez that would render half of Istanbul an island, an airport that will be the world's busiest, and a giant mosque atop an Istanbul hill.

Atomic power stations are also on the drawing table, while a third bridge over the Bosphorus - whose construction has already felled 1 million trees - is under way.

The plans have fired up Mr Erdogan's opponents, who dub them "pharaonic projects" and say they are a symptom of an increasingly authoritarian style of government, and warn of environmental catastrophes in one of world's most earthquake-prone nations.

They accuse Mr Erdogan, still broadly popular after 10 years in power, of bypassing city planners and bulldozing history to make way for pet projects in an ancient city that was once the capital of the Byzantine Empire, before the 1453 Islamic conquest after which it became the centre of Ottoman power.

The mega projects would add at least $180 billion to Turkey's foreign debt stock, according to analyst Atilla Yesilada, further swelling an already massive current account deficit, which the IMF says may reach 7 per cent of economic output this year.

"Rather than having a social utility, some of these seem to be legacy projects: Erdogan trying to leave his mark on the Turkish landscape and history," Mr Yesilada said. "It is like pharaohs building more pyramids to their names."

Worries over proximity to fault line

The Union of Chambers of Turkish Engineers and Architects warned the Marmaray, set on a silty seabed 20 kilometres from the active North Anatolian Fault, is at risk in case of a large earthquake, which geologists predict may strike within a generation.

But transport minister Binali Yildirim described the Marmaray as the "safest structure in Istanbul," with its free-floating structure designed to withstand an earthquake with a magnitude of 9.

Interlocking floodgates would seal off each section should water break through into the tunnel.

It is expected that the Marmaray will reduce car traffic by 20 per cent in Istanbul - among the world's most congested cities - when it eventually carries 1.5 million people a day.

Murat Guvenc, director of the Urban Studies Research Centre at Istanbul Sehir University, said the tunnel will essentially shrink Istanbul, a sprawling metropolis of 15 million people.

"The historical peninsula has remained intact for 25 centuries, like the eye of the storm, because of the natural barriers of the Golden Horn and Bosphorus waterways," Mr Guvenc said. "The Marmaray removes those boundaries."

Construction of the tunnel on the European side yielded a Byzantine port with more than 13 shipwrecks and thousands of other relics that date back as far as 8,500 years.

The finds nearly doubled the project's duration and prompted UNESCO, the United Nations' cultural arm, to voice concern about threats to the peninsula, a World Heritage site.

The government will open an "archaeological park" at the Yenikapi subway station to showcase relics. Station walls are decorated in a Hellenic theme with amphoras and galleons.

"Had it been up to the archaeologists, this project would have never finished," Mr Yildirim said.